Parallax GPS receiver has a timing source

Hello all,

does anyone know if it is possible to use a Parallax GPS receiver to generate interrupts at specific instants?

For example, call an interrupt function once every absolute second?

I believe the Parallax unit has an output for the one second clock.

Many (but not all) GPS units have a pulse per second (PPS) output in addition to the serial I/O. Not sure about the Parallax model in particular, but PPS is probably a good search keyword to look for in the datasheet.

-j

I believe the Parallax unit has an output for the one second clock.

+1

This is what I was able to find for the GPS module:
http://www.polstargps.com/Support/Spec/PMB-248-648.pdf

unfortunately, it does not seem to have the pps pin. Only RS232 and TTL comms.

That's correct.

You may be able to get 1Hz out of the GPS some other sneaky way, like reducing the GPS output to one sentence, tying the TTL TX line to an interrupt, and trigger off the start of the NMEA sentence (that's sent once per second). You'd need to disable the interrupt or otherwise ignore it while the bits of the sentence are being transmitted. If the timing is not so critical, reduce it to one sentence and read characters, triggering off the start sentinel ($) or line terminator characters returned by serial.read().

Some RTC devices (e.g. DS1307) have the capability of putting out a 1Hz square wave. They're quite a bit cheaper than a GPS, too.

-j

Thanks for the suggestion kg4wsv. I will try just that.

Unfortunately, I really need to use GPS clock because I need to synchronize several arduinos in remote locations at sub-milisecond level with universal time.

If cost is an issue you may want to consider using a WWV or DCF clock receiver:
http://gonium.net/md/2006/11/05/arduino-dcf77-radio-clock-receiver/

I really need to use GPS clock because I need to synchronize several arduinos in remote locations at sub-milisecond level with universal time.

I think I'd choose a better GPS for the job, then. You might even find one that's both cheaper and has PPS output.

The Garmin GPS18LVC is a mouse-type GPS that has 5V TTL level serial IO and a TTL PPS output. It requires regulated 5V in (the arduino should be able to power it, depending on what else you have powered from the arduino board). Full retail price is only a few US$ more than the Parallax.

-j

Thanks for the suggestion mem.
However, I live in the Azores islands and DCF77 is far to weak here.
I do not think that the local government provides this type of service. But it would certainly be usefull.